(1) Field of the Invention
The present invention pertains to the field of compound archery bows. More particularly, it relates to the eccentrically mounted pulleys or wheels, also known as "cams", which are positioned at the end of the archery bow limbs and/or the bow riser or a combination thereof. The present invention is designed, like the prior art systems, to reduce the holding force at the maximum bowstring draw position, but the present invention differs from the prior systems in the following respects: by attaining and maintaining a high amount of stored energy; by increasing the "letoff" greater than any prior art system; and by offering variable or adjustable draw length with a precise stop at the full draw position. "Letoff" is defined as the amount of the reduction of the holding force exerted by the archer at the fully drawn position. Specifically, the present invention accomplishes four main advantages not accomplished by the prior art systems: (a) it achieves as high as 90% of the theoretical maximum stored energy; (b) it achieves adjustable letoff of 90% or greater instantaneously at the point of full draw; (c) it achieves variable or adjustable draw length to fit the entire range of users; and, (d) it provides for a precise full draw position set for each individual user.
(2) Description of the Prior Art
Since the advent of the compound archery bow exemplified by the Allen bow disclosed in Allen U.S. Pat. No. 3,486,495 issued Dec. 30, 1969 there has been an increasing adoption and use of the compound bow by thousands of archery enthusiasts.
The following patents illustrate eccentric pulley systems that are considered to be the state of the art prior to the present invention:
Rickard U.S. Pat. No. 4,203,412 issued May 20, 1980 represents the best of the prior art for achieving maximum letoff with minimal loss of potential energy in the last 10% of the draw force curve. The disadvantages of this system are that: it uses a complex trigger release mechanism; it does not incorporate many of the desirable features found in the more traditional compound bow; and while exhibiting less potential stored energy loss over the final portion of the draw better than any other available system it still loses 50% of the potential energy possible over the last several inches of the draw force curve.
Simonds et al U.S. Pat. No. 4,461,267 issued July 24, 1984 and Miller U.S. Pat. 4,519,374 issued May 28, 1985 represent the best of the prior art for maximizing stored energy as represented by the area under their draw force curves. The disadvantages of these systems are that: they are only storing 80% of the maximum potential energy available; they are locked into a low letoff configuration which make them uncomfortable to hold at full draw; they do not have a precise point at which the user comes to full draw; they require additional components to alter draw length; and, in the case of the Simonds et al system, it represent a complex mechanical system which is not acceptable to many users.
Jarrett U.S. Pat. No. 4,512,326 issued Apr. 23, 1985 represents another system which seeks maximum letoff as a primary consideration. The disadvantages of this system are that: it loses potential energy in the front end of the draw force curve; it loses potential energy approaching the full draw position as in the Rickard system; it does not provide for adjustability of the draw length to fit individual users; and, the combination of the stiff limb/flexible limb gives it an appearance which is not readily acceptable to the general user.
Jennings U.S. Pat. No. 4,562,824 issued Jan. 7, 1986 represents a state-of-the-art system for altering draw length with minimal tools, without dismantling the bow, and without changing the bow peak draw weight. The disadvantages of the system are: a complex mechanical system is employed; draw length adjustment is available only in predetermined increments; additional pieces are required; the system does not provide for a precise maximum draw length point; and, the appearance of the system is unusual which leads to consumer reluctance.
In contrast to the foregoing prior art systems, the present invention provides an improved eccentric compound pulley system, which we have named the "moment transfer pulley system", which retains all of the traditional advantages of a compound bow, but which will also store a greater amount of potential energy in the limbs while decreasing the force necessary to hold the bow at full draw by a greater amount than any prior art system. The release of this higher amount of stored energy results in a faster arrow and a steadier arrow due to the maximum uniform force which is imparted to the arrow.
A further advantage of the moment transfer pulley system is that it provides a variable or adjustable draw length to fit individual users and it provides a precise full draw length stop point to fit each individual user.